ID: ae1e3 No.575
Warm water works fine for warming up. I would highly recommend getting a USB warmer, however.
Without warming up, there's a brief moment of cold during insertion, which can be a bit awkward-feeling. This will go away after a few seconds as you warm up the material with your dong. If you warm it in warm water, you can bring it to roughly skin temperature at the beginning, which is good.
However, with a USB warmer, you can bring it up to above skin temperature, so that there's an intense warming sensation when you insert your dong. It's /very/ pleasant - if you've ever put on a t-shirt right after warming the inside with a blow dryer, or right after it's out of the laundry and still warm, it's a similar sensation. The warmth goes away within a few seconds, but it's still enjoyable enough that I'd highly recommend a USB warmer if you think you'd like it.